Diffraction gratings disperse light. A blazed diffraction grating disperses light into a single order. The diffraction gratings are used in devices such as spectrometers and scanning monochromators. The diffraction gratings are also used in WDM (wavelength division multiplex) optical communication. In the WDM optical communication, multiple wavelengths of light each carry a communication signal. Each of the multiple wavelengths of light forms a WDM channel. In DWDM (dense WDM) optical communication, a subset of the WDM optical communication, the WDM channels are spaced close together. A typical DWDM application operates at a wavelength band about 1,550 nm, has 90 channels, and has spacing of about 0.4 nm between adjacent channels. In the WDM optical communication, the diffraction gratings are used to demultiplex and to multiplex the WDM channels.
The diffraction gratings used in the WDM optical communication suffer from several deficiencies. A first deficiency is that the diffraction gratings produce a relatively small angular dispersion for adjacent WDM channels. In order to distinctly separate the WDM channels, the relatively small angular dispersion leads to a need for a long optical path. A second deficiency is that reflective diffraction gratings having a fine ruling density tend to exhibit polarization dependent loss, which can exceed 1 dB.
What is needed is a method of dispersing light that disperses light with a dispersion which is greater than what is available with a blazed diffraction grating.
What is needed is a method of dispersing light that disperses light with a dispersion which is greater than what is available with a blazed diffraction grating and which also reduces polarization dependent loss.